塔河油田奥陶系古岩溶洞穴充填物时代鉴别特征及其储集意义

Ordovician palaeokarst caves in the Tahe oilfield:Burial age of cave fills and its implication for hydrocarbon reservoirs

洞穴充填物的充填时代分析能够帮助认识洞穴的形成过程、充填结构序列和储集空间发育规律。塔河油田奥陶系岩溶洞穴受到物理充填作用的显著影响,其充填物的时代鉴别特征及其对储集空间发育的影响缺乏系统的分析。基于塔河油田奥陶系地质背景、岩心、测井、测年及产能资料的分析,认为:(1)灰绿色层理构造钙质砂岩和泥质粉砂岩为搬运充填成因,与裂缝共生的灰绿色泥质粉砂岩为渗流充填成因,它们的充填时代为海西早期,杂色砾岩的砾石成分复杂,磨圆分选较好,颗粒支撑,也为搬运充填成因,但充填时代为加里东晚期;(2)角砾岩的角砾成分一致,磨圆分选差且被灰绿色渗流泥充填,为海西早期洞穴掩埋时垮塌充填的结果,无渗流泥充填的角砾岩,裂纹化且多含方解石或沥青,多发育在洞穴顶部,为深埋藏阶段垮塌作用的结果;(3)岩溶洞穴缝洞储集体控制因素十分复杂,洞穴掩埋期的搬运、渗流与垮塌作用多产生泥质充填,储集性差;深埋藏期垮塌作用,产生次级裂缝且无泥质充填,储集性好,洞穴顶部及上部为有利储层发育位置。本研究为岩溶洞穴形成过程和油气藏高效开发提供有价值的参考。

The burial age of cave fills can provide valuable insights into the formation of ancient caves and the cave filling order and cave reservoir development.The Ordovician palaeokarst caves in the Tahe oilfield are significantly impacted by cave filling,but the burial age of cave fills and its implication for the hydrocarbon reservoir development in the oil field have not been systematically analyzed.Here we present findings on the cave deposits based on analyses of the geologic setting of the oil field as well as core,well-logging,burial history and productivity data.We found the layered,gray-green calcareous sandstone and argillaceous siltstone were deposited via transport filling in the Early Hercynian and fracture-associated,gray-green argillaceous siltstones via seepage filling in the same era;whilst well-rounded/sorted,multicolor conglomerate fills,with complex rock composition and sandy matrix,were deposited in the Late Caledonian by transport filling.Meanwhile,poorly-sorted collapse breccias,chemically homogeneous,mixed with grey-green mudstone via seepage filling,were deposited in the Early Hercynian during early burial;whilst fractured,calcite/oil-bearing collapse breccias,mainly on cave roof,without seepage filling,were deposited in the late burial stage.The reservoir controlling factors of fracture/cavity reservoirs of karst caves were very complex,where cave fills deposited in the early burial stage via transport,seepage and collapse filling were mostly mudstones,with poor reservoir quality;whereas secondary fractures formed during late burial from collapse settlement without mud fill greatly improved reservoir quality,and the upper/top parts of karst caves were favorable for reservoir development.This research provided a valuable reference for the understanding of karst cave formation and exploration of high-quality hydrocarbon reservoirs.